Which description does not guarantee that a trapezoid is isosceles?
a) it has congruent diagonals
b) both pairs of base angles are congruent
c) it has congruent bases
d) it has congruent legs

The correct answer among all the other choices is C) it has congruent bases. This description does not guarantee that a trapezoid is isosceles. Thank you for posting your question. I hope this answer helped you. Let me now if you need more help. 

The answer is B congruent angles